1. Field of the Invention
The present invention relates to a multi-beam optical scanning device and an image forming apparatus having the multi-beam optical scanning device.
2. Description of the Related Art
In recent years and continuing, an image forming apparatus, such as a digital copier and a laser printer, can form images by optical scanning at increasingly high density, and perform optical scanning at increasingly high speed so that an image forming apparatus capable of forming images of higher density and optical scanning at higher speed is required. As an effective measure for increasing the optical scanning speed, a multi-beam scanning technique, which involves scanning of a scanning surface by plural light beams at the same time, is being developed and is approaching practical use. By using the multi-beam scanning technique, it is possible to perform high-speed writing without raising the rotational speed of a polygon motor, and this can reduce power consumption.
In addition, a technique of using resin lenses has also been developed for the purpose of reducing cost of the system. When using resin lenses, it is important to ensure an optical scanning device is not adversely influenced by changes of the ambient temperature.
Even when using glass lenses, the surface curvature, thickness, and refractive index of the lens may change due to the temperature variation of the circumstance, and the refractive index of the lens may also change due to variation of the wavelength of a semiconductor laser, which serves as a light source. Due to these changes, the size of light spots becomes large, and this limits the image density. When using the resin lenses, the changes of the surface curvature, thickness, and refractive index of the lens caused by the ambient temperature variation, and the change of the refractive index of the lens caused by variation of the wavelength of a semiconductor laser are larger than the changes for the glass lenses. Hence, in an optical scanning device using the resin lenses, the influence of the ambient temperature variation on the image density is large.
To solve the problem, for example, Japanese Laid Open Patent Application No. 2002-214556 (hereinafter, refer to as “reference 1”) discloses a technique in which at least three lenses are combined in an optical system in front of a deflector. In addition, Japanese Laid Open Patent Application No. 11-223783 (hereinafter, refer to as “reference 2”) discloses a technique in which a diffracting surface is provided on a scanning lens for correction. Further, the above-mentioned reference 2, and in addition Japanese Laid Open Patent Application No. 2004-126192 and Japanese Laid Open Patent Application No. 2003-337295, (hereinafter, refer to as “reference 3”, “reference 4”, respectively) disclose techniques in which a resin lens having a diffracting surface is disposed in front of a deflector to reduce the change of the beam spot size caused by the temperature variations.
However, in the technique shown in reference 1, when the number of the lenses rises, the cost of the device rises. In addition, since at least one glass lens is required, this also increases the cost.
In the technique shown in reference 2, since the region through which the light beam passes is broad, it is time-consuming to fabricate the diffracting surface, and this also increases the cost.
In the technique shown in reference 3, since a shift of the beam spot position caused by the temperature change of the optical system behind the deflector (that is, a scanning optical system) is not considered, when a resin optical element is used in the scanning optical system, the shift of the beam spot position cannot be reduced to be sufficiently small.
In the techniques shown in references 2 and 4, since a change of the arrangement of optical elements in the first optical system is not considered, when a resin optical element is used in the scanning optical system, the shift of the beam spot position cannot be reduced to be sufficiently small.
In addition, since plural light sources are used in an optical scanning device operating in the multi-beam scanning scheme, a difference of wavelengths between the plural light beams of the light sources is inevitable. Since the size of light spots changes in response to the light beams having different wavelengths, the wavelength difference of the light beams also affects the image density.
Specifically, since the power of a diffracting surface strongly depends on the wavelength of the incident light beam, in an optical system utilizing diffracting surfaces, if the power of the diffracting surfaces and refractive surfaces are not arranged appropriately, beam spot position shifts between adjacent scanning lines and beam spot sizes differ from each other. This degrades image quality.
As described above, the aforesaid reference 2, reference 3, and reference 4 disclose methods for correcting the ambient temperature variation, but these references do not consider the problem of the wavelength difference of the plural light beams. Hence, even though appropriate correction can be made for one light beam, sometimes, appropriate correction cannot be made for other light beams having different wavelengths.